The present invention concerns a process and an apparatus for the control of the degree of cleanliness of metallic surfaces of containers, equipment, or pipelines. The invention is especially of interest in areas in which the cleanliness is to be checked at inaccessible, or not visible locations, such as the interior walls of containers, equipment, or pipes of that kind.
Because of the impossibility of a visual inspection, or the bother of checking such interior surfaces, control measures are often even omitted entirely in practice. In those cases, cleanliness is simply assumed on the basis of the experience that, in general, after cleaning measures under defined conditions, the container, equipment, or pipe should be clean. In those cases in which one did not want to do without a cleanliness control, the following measures have been taken:
Investigation of the washing liquid after emerging from the equipment to be cleaned; this liquid is evaluated either visually, or by exact analytical methods. PA0 Determination of the contamination by back-weighing ("Seifen, Fette, Oele, Wachse" 1953, pp. 488-489, 514-516, 540-542, 568-569, 597-599, 622-625, 645-647). PA0 Recording of the radioactive residual radiation of the contamination mixed with tracer substance ("Fette, Seifen, Anstrichmittel" 80 (1978), pp. 43-50, 80-85, 118-124).
The disadvantages of the foregoing measures are obvious: either the degree of cleanliness can only be roughly estimated or cannot be determined in the region of small quantities due to the limits of analytical measuring methods. In addition, these measures are cumbersome, time consuming, or even hazardous (the latter e.g. in the case of radioactive recording). Cumbersome and time-consuming are e.g. those cleaning processes where absolute cleanliness of the interior walls is important, as in the storage or processing of food or beverages. Here, in order to be safe, the cleaning process has to be greatly extended so as to guarantee absolute cleanliness.
A method is furthermore known which attempts to determine whether the metal surface is freed from contamination during the cleaning process via the contribution of this exposed metal surface to the conductivity of an electrolytic system. Thus, according to "Deutsche Textiltechnik" 10 (1960), pp. 589-593, a standardized fatty contamination is applied to two platinum wires and the decline in surface coverage is traced by the discontinuous measurement of the electrolytic conductivity between two platinum wires, the resistance being measured with a Wheatstone bridge. It is mentioned that the desired ideal arrangement for this method is an "autographic measuring bridge". Because of the limitation to liquids with badly conducting anionic, or even non-ionogenic detergents, it is necessary to add an electrolyte, because of which the cleaning result may be influenced (e.g. in the case of cleaning agents containing polyphosphate). No mention is made of an extension of the process to other metals than platinum, such as the stainless steel or aluminum customarily used in container, instrument and pipe designs.
There was therefore a need for a process to control the cleanliness of metallic surfaces, which is not afflicted with the above-mentioned disadvantages and which, above all, can be used quickly, continuously, reliably and universally for different metals and a plurality of cleaning agents and cleaning processes for the most varied contaminations.